1. Field of the Invention
The present invention relates to a deformable mirror device whose mirror surface can be deformed.
2. Description of the Related Art
In optical disc apparatuses for performing recording and/or reproduction with respect to an optical disc recording medium, laser light from an objective lens is focused on the recording layer of the optical disc recording medium to thereby perform signal recording/reproduction.
It is known that when radiating laser light via the objective lens in this way, a spherical aberration occurs depending on the thickness of a cover layer (cover thickness) of the optical disc recording medium from the laser-light-incident side surface (recording-side surface) to the recording layer. That is, since the optical system of an optical disc apparatus including an objective lens is designed in such a way as to minimize spherical aberration in accordance with the value of a cover thickness assumed for the corresponding optical disc recording medium, a spherical aberration occurs if the cover thickness differs from the assumed value. In the case where the recording layer is made of a single layer, for example, it is known that a spherical aberration occurs at a portion of the recording layer where cover thickness variations exist.
In recent years, some optical disc apparatuses are provided with multiple recording layers in order to achieve higher recording density. In the case where multiple recording layers are provided in this way, since the cover thickness is naturally made to differ among the respective recording layers, a spherical aberration occurs when performing recording/reproduction with respect to a recording layer other than the recording layer that serves as a reference.
When a spherical aberration occurs, the imaging performance deteriorates and hence the signal recording/reproduction performance also deteriorates. Some means for correcting such a spherical aberration is thus required.
Examples of the related art techniques aimed at correcting such a spherical aberration resulting from variations in the cover thickness of an optical disc recording medium include one in which correction is performed by deforming the profile of a mirror that is provided to guide laser light to an objective lens in the optical system (see Japanese Unexamined Patent Application Publication No. 5-151591 and Japanese Unexamined Patent Application Publication No. 9-152505).
That is, the deformable mirror disclosed in Japanese Unexamined Patent Application Publication No. 5-151591 mentioned above includes a deformation plate having a mirror surface formed on its front side, and piezoelectric actuators for pressurizing several locations on the back side of this deformation plate. By changing the voltage applied to each piezoelectric actuator, the above-mentioned mirror surface is changed to a desired profile that enables correction of spherical aberration.
Further, in the deformable mirror disclosed in Japanese Unexamined Patent Application Publication No. 9-152505, after forming a flexible member having a mirror surface formed on its front side, and a reference surface of a predetermined profile below the flexible member, the flexible member is adsorbed onto the reference surface or the adsorption is released, thereby obtaining two desired profiles. That is, the profile of the reference surface is set such that the profile of the mirror surface upon adsorption becomes a profile that enables correction of spherical aberration, thereby making it possible to correct spherical aberration.